Steel sheets including silicon have good magnetic properties and are thus widely used as electric steel sheets. For example, silicon steel sheets are used as materials for the cores of transformers, electric motors, generators, and other electronic devices, and in this case, silicon steel sheets are required to have good magnetic properties. Particularly, silicon steel sheets are required to be effective in reducing energy loss due to current environmental and energy problems. Concern about environmental and energy problems may be related to magnetic flux density and core loss. That is, as the density of magnetic flux is increased, the size of cores can be reduced to make electric devices smaller, and as core loss is reduced, energy loss is also reduced.
Core loss causing energy loss includes eddy current loss and hysteresis loss. As the frequency of an alternating current (AC) current increases, the amount of eddy current loss increases. Eddy current loss occurs in the form of heating when a magnetic field is applied to a core, and silicon is added to a core to reduce eddy current loss in the core. If the content of silicon in steel is increased to 6.5%, magnetostriction causing noise does not occur (0%), and the permeability of the steel is maximized. In addition, in the case that the content of silicon in steel is 6.5%, the magnetic properties of the steel may be markedly improved. Therefore, high silicon steel having good magnetic properties may be used in high-value electrical devices such as inverters and reactors for new renewable energy power stations, induction heaters for gas turbine power generators, and reactors for uninterruptable power supplies.
High silicon steel sheets including a silicon content of 6.5% are excellent in terms of magnetic properties. However, as the silicon content of steel sheets is increased, the steel sheets are increased in brittleness and markedly decreased in elongation properties. Thus, it is known that silicon steel sheets having a silicon content of 3.5% or greater are practically impossible to manufacture using general cold-rolling methods. That is, high silicon steel sheets known as having good magnetic properties are not manufactured by cold-rolling methods due to inherent limitations of cold-rolling technology. Thus, research into new technology has long been conducted to overcome limitations of cold-rolling methods.
Since it is difficult to manufacture high silicon steel sheets having good magnetic properties through a general hot-rolling process and a general cold-rolling (or warm-rolling) process, there have been attempts to manufacture high silicon steel sheets through other methods.
Methods currently known as techniques for manufacturing high silicon steel sheets are casting methods in which high silicon steel sheets having a final thickness are directly manufactured through a casting process using a single roll or a pair of rolls. An example of such a method is disclosed in Patent Document 1. In such methods, however, it is very difficult to control the shape of a cast plate. Particularly, if molten steel is directly cast as a plate having a final product thickness, the surface of the plate may be very rough and easily cracked, and thus it is difficult to obtain plates having improved magnetic properties using such a direct casting method. In addition, such a direct casting method is not suitable for commercial mass production because of uneven thicknesses of cast plates. Patent Document 2 discloses a so-called clad method in which high silicon steel covered with low silicon steel is rolled. However, the disclosed method has not yet been commercialized.
In addition, Patent Document 3 discloses a powder metallurgy technique for making a high silicon steel block as a substitute for a high silicon steel sheet. Although pure iron powder cores, high silicon steel powder cores, and Sendust powder cores are used in combination, such cores have soft magnetic properties inferior to those of high silicon steel sheets because of characteristics of powders they are produced from.
According to current mass-production technology for manufacturing high silicon steel sheets having a silicon content of 6.5%, a chemical vapor deposition (CVD) method is used to diffuse SiCl4 into a steel sheet having a silicon content of 3% during an (diffusion) annealing process. Many examples of the technology such as that disclosed in Patent Document 4 are known. According to the technology, however, toxic SiCl4 is used, and it takes a significant amount of time to perform a diffusion annealing process.
In addition, there have been attempts to manufacture thin high silicon steel sheets in laboratories by a so-called warm-rolling method in which the temperature of a rolling process is increased. If slabs are manufactured through a general continuous casting process, the slabs are heated to 1100° C. or higher for several hours in a reheating furnace before a hot-rolling process, and at this time the slabs may crack due to differences in temperature between the surfaces and centers thereof. In addition, when the slabs are removed from the reheating furnace and hot-rolled, the slabs may fracture. For example, FIG. 1 illustrates 6.5%-Si steel melted in a 50-kg vacuum induction melting furnace, formed into a 200-mm slab by milling, heated to 1100° C. for one and a half hours under an argon (Ar) atmosphere, and immediately hot-rolled. The slab fractured during hot-rolling. This technique of increasing rolling temperature may improve rolling characteristics of steel but causes many other problems during a hot-rolling process.
(Patent Document 1) Japanese Patent Application Laid-open Publication No. S56-003625
(Patent Document 2) Japanese Patent Application Laid-open Publication No. H5-171281
(Patent Document 3) Korean Patent No. 0374292
(Patent Document 4) Japanese Patent Application Laid-open Publication No. S62-227078